The invention relates to an electron multiplier assembly such as that used in a photomultiplier tube, and particularly to an anode member for an electron multiplier assembly that is resistant to electrical shorts.
The growing demand for petroleum products has placed increasing emphasis upon the need for improved oil exploration techniques. As the supply of easily obtainable oil dwindles, exploration has had to move to more remote geographical areas and to deeper fields. One technique for accurately determining the location, size and yield of an oil field is by oil-well logging. Logging is a term given to the method of determining the mineral composition and structure of the geological media along very deep holes. Sensitive probes, or sondes, are used to determine the lithology, i.e., the character of the rock formation, including the density of the media along the bore hole. The bole holes are typically thousands of feet deep and may extend to about twenty thousand feet. Temperature increases with bore hole depth and the temperature in a twenty thousand feet deep hole may range between 100 .degree. to 250.degree. C. In logging such a hostile environment, the sondes, which include a radioactive gamma ray source such as cesium 137 and a detector comprising a sodium iodide crystal and a photomultiplier tube, are subjected to shock and vibration in addition to high operating temperatures.
Photomultiplier tubes used for oil-well logging are preferably small and rugged. The RCA C33016G photomultiplier tube has a 25.4 mm diameter and a length of about 60 mm. It is well known in the art that the deleterious effects of shock and vibrations can be minimized by using stiff, short support leads, extending through the base of the tube and connected directly to the active tube elements such as the anode and the dynodes, to resist and quickly damp vibrations. However, the stiff support leads expand as the sondes are lowered into the bore hole and the temperature increases. This thermal expansion causes flexing of the tube elements which may result in electrical shorts. In the RCA C33016G tube, the spacing between the anode and the ultimate dynode is about 0.1 mm (0.004 inch). Such close spacing is required in order to provide fast and efficient anode response characteristics. As shown in FIG. 5, a conventional anode, A, is disposed within the substantially elliptically-shaped ultimate dynode, D.sub.N. Despite the fact that the longitudinally-extending edges of the anode are curled to stiffen the structure, electrical shorts caused by environmentally-induced anode movement frequently occur between the ultimate dynode and the anode.
A somewhat more rigid type of anode structure in which the anode member comprises a thin, solid, flat plate disposed substantially parallel to and just outside of the main path of the electron flow from the penultimate dynode to the ultimate dynode is shown in U.S. Pat. No. 2,868,994 issued to Anderson on Jan. 13, 1959. In the Anderson patent, the anode does not intercept electrons traveling in the main path; however, the anode presents a favorably large surface to the cylindrically-shaped ultimate dynode which is so curved that the secondary electrons emitted therefrom are focused onto the anode. Furthermore, the solid flat plate prevents electron orbiting or oscillation which commonly occurs with rod-like and grid-type anodes. Unfortunately, the anode structure of the Anderson patent is impractical for use in small electron multipliers, such as that used in the RCA C33016G, since the elliptically-shaped ultimate dynode, dictated by the size constraints and environmental requirements of a small tube, has a limited opening for electrons from the penultimate dynode and the Anderson anode would intercept an unacceptably large percentage of the electron flow to the ultimate dynode. Scaling down the dimensions of the Anderson anode to increase the electron flow to the ultimate dynode would weaken the anode thereby increasing the possibility of anode shorts. Additionally, an anode structure, such as the Anderson anode, which is disposed substantially parallel to the main path of the electron flow and extends beyond the ultimate dynode, creates the possibility of an electrical short between the anode and the penultimate dynode.
A conventional anode structure, such as a grid-type anode comprising a pair of parallel support rods having a fine wire mesh wound thereon, is shown in U.S. Pat. No. 4,002,735 to McDonie et al., issued on Jan. 11, 1977. The grid-type anode is unacceptable for use in a small electron multplier subjected to hostile environments since in addition to the electron oscillation problem common in grid-type anodes, wire wound anodes frequently develop electrical shorts to the ultimate dynode because of broken mesh wires, or mesh wire sag caused by flexing of the support rods induced by thermal changes in the anode support lead which is connected to the support rods.